The present invention relates to conduits and couplings in general, and more particularly to conduits for carrying materials at different temperatures.
There are many manufacturing processes where materials of different temperatures must be conveyed through the same conduit. Due to the phenomenon of thermal expansion, solid conduits will become longer when temperature is increased, and shorter when temperature is decreased. This effect can become especially pronounced when multiple fluids of different temperatures are conveyed through coaxial conduits. In some food processing plants, for example, temperature-sensitive food product is conveyed through an interior conduit and is maintained within a suitable temperature range by a coolant fluid flowing through an exterior volume located between the interior conduit and an exterior conduit. At regular intervals the food product is flushed from the interior conduit and a hot fluid is coursed through the interior conduit to clean it in place. This cleaning may take place while the coolant is still present in the exterior conduit. Unless very expensive materials such as Invar metal (64FeNi) are used, the difference in temperature will cause such a change in dimension of the inner pipe and the outer pipe, that failure of the materials may result. Stainless steel, a common material used in sanitary food processing, has a thermal coefficient of expansion of about 31×10−6 which will cause more thermal stress depending on the temperature differential and the length of the two sections held at different temperatures. As the stress to which the conduit is exposed approaches the yield strength in any part of the duct structure, the number of heating and cooling cycles before leaking will decrease. When the stresses are in the plastic range for the duct material, fatigue cracking occurs very rapidly.
In many sanitary situations cooled conduits may be tens or even hundreds of feet long which makes accommodating thermal expansion a critical issue. Thus connections between the hot and cold portions of the conduit may become fatigued or ruptured, especially with repeated cycling of the conduit assembly, greatly shortening the useful lifetime of the conduit assembly or requiring frequent downtime and repairs. One known approach employs a bulky and costly attachment assembly.
What is needed is a conduit assembly for multiple fluid flows which can accommodate the thermal cycling over an extended period of time.